Although rotary inertial devices (RIDs) have been used in resistance training for many years, there is still limited knowledge of the specific biomechanics that distinguish them from traditional resistance training (TRT) methods. The aim of this study is to compare the serial data of force, velocity, and displacement over time in half-squats performed with both devices when the intensity is based on the concentric mean propulsive velocity (MPV). A total of 20 experienced subjects completed 3 sets of 6 half-squats using both RID and TRT. To ensure a similar load intensity, the concentric phase was matched according to the MPV. Measurements of vertical ground reaction force (vGRF), velocity, and displacement were taken for each repetition of the half-squat. The results showed that TRT exhibited a higher vGRF than RID during 0-57% of the concentric phase but a lower vGRF during 74-93% (p < 0.001). Eccentric vGRF was also higher for TRT throughout much of the eccentric phase (0-13%, 38-54%, and 68-100%, p < 0.001). Rotary inertial device demonstrated faster vertical velocity than TRT during 31-52% of the concentric phase and 1-40% of the eccentric phase (p < 0.001). However, during the latter part of the concentric phase (72-99%), TRT exhibited faster vertical velocity compared with RID. In addition, TRT resulted in a higher vertical position than RID at the end (67-100%) of the concentric phase (p = 0.036). Coaches should be aware of these biomechanical differences when prescribing resistance training with RID or TRT, as even with similar loads, distinct patterns in vGRF and velocity over time can lead to different effects on the athlete.
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